DETROIT — This year you absolutely must fill out your federal income-tax return before deciding how you're going to spend your refund.

Why? Because you could be coming up short or even owing the government money, thanks to some quirkiness in a "Making Work Pay" tax credit.

"It's an odd computation," said James Jenkins, president of Jenkins & Co., a tax firm in Southfield, Mich.

Most likely to encounter some tax-time troubles are people who worked two or three jobs in 2009, two-paycheck couples, someone getting Social Security but also earning wages and people who work but also have outside income — such as dividends, interest or capital gains — that can drive up modified adjusted gross income.

"People might have too little withholding," said Scott E. Weiner, senior tax analyst from the tax and accounting business of Thomson Reuters.

Remember back in April when paychecks were tweaked to seem a tiny bit bigger? Perhaps you barely noticed, but millions of people started seeing a little extra money in their checks after withholding tables changed as part of the federal economic-stimulus package.

The American Recovery and Reinvestment Act of 2009 provides married couples filing jointly with a maximum $800 refundable tax credit under the Making Work Pay plan; singles receive a credit of up to $400. The credit applies to last year and this year.

Now, during tax time, many taxpayers will need to fill out a Schedule M to calculate the exact credit that they deserve under Making Work Pay.

There's also a Government Retiree credit of up to $250 — $500 for joint filers if both spouses qualify — for people who received a pension or annuity payment in 2009 for service performed for the U.S. government or any state or local government if the service was not covered by Social Security.

But — and this is key — you cannot take the $250 credit if you and your spouse both received a $250 economic-recovery payment in 2009. Those payments went out to various groups, including people receiving Social Security benefits.

This tax season, much of the buzz will be about that Making Work Pay credit and how some households will get an unsettling surprise after they run the exact numbers.

As Jenkins noted, the withholding tables do not reflect every family's financial situation.

"Those tables don't know that your husband works — or even has a job," Jenkins said.

The Making Work Pay credit does not apply to all taxpayers; it is reduced or eliminated for higher incomes.

The credit is phased out for a married couple filing a joint return whose modified adjusted gross income is between $150,000 and $190,000 and for other taxpayers whose modified adjusted gross income is between $75,000 and $95,000.

CCH principal tax analyst Mark Luscombe noted estimates showing that more than 15 million taxpayers may have received extra money in their paychecks last year reflecting the new withholding tables but won't be entitled to the full Making Work Pay credit.

The U.S. Treasury Office of the Inspector General for Tax Administration estimated that more than 1.2 million taxpayers may have to repay some or all of the Making Work Pay Credit and could be assessed the estimated tax penalty or an increased estimated tax penalty as a direct result of the credit. The IRS has strongly disagreed with that calculation, estimating that only 68,000 taxpayers could be faced with an estimated tax penalty and that the penalty is easily waived for 2009 returns.

The instructions for Form 2210, which covers underpayment of an estimated tax penalty, indicate that, if the underpayment is because of the adjustments in the withholding tables for the Making Work Pay Credit, a taxpayer is entitled to a waiver of the penalty.

Luscombe noted, though, that you would still have to file Form 2210 to claim the waiver.

He offered an example of how this could hit some higher-income workers. Say a husband earns $120,000 and his wife $70,000. Each income is separately below the married-filing jointly cutoff for the Making Work Pay Credit.

As a result, Luscombe said, each spouse would have seen an extra $600 in their paychecks under the revised withholding tables.

But their combined incomes are too much to qualify for the $800 credit for joint filers.

As a result, their federal income-tax refund will be short by $1,200 or they may even owe money, depending on the specifics of their return.

And yes, it is also possible that even a married couple who both work outside the home and qualify for the $800 tax credit still could owe a few hundred dollars because too little was withheld based on the tables.

Taxpayers who face a headache this year might want to consider visiting their human-resources office now to have extra money withheld in 2010. The Making Work Pay Credit also applies to this year.